In window construction, hardware refers to the components that enable the window to be opened and closed. It therefore comprises the locking mechanism on the opening side of the window sash and the corresponding keeps in the frame, as well as the hinges on the opposite side of the window sash. All of the mechanics that allow a window sash to be opened, and in the case of a tilt-and-turn window to be turned or tilted, are also part of the hardware. In professional terminology, the window handle used to operate the hardware is not counted as part of the hardware itself, but is often also "meant" when hardware is spoken of.

The term spacer refers to the window components that form the transition between the glass surface and the frame profile. They are also responsible for the distance between the individual glass panes of a multiple glazing unit, hence the name. In order to achieve a good U-value and to significantly minimise heat losses at the edge seal, modern spacers are no longer made of aluminium, but of flexible structural foam, composite plastic or plastic-coated stainless steel. These are used when a window is to be produced with what is known as a "warm edge". Passive house standard windows are only possible with such modern spacers.

In window and door construction, frame depth refers to the depth of the frame structure required within the masonry. It is stated in millimetres and, on modern triple-glazed windows, is at least 70 millimetres, though often as much as 90 millimetres. Older windows usually have a shallow frame depth. When replacing windows, it is important to bear in mind that modern, multiple-glazed windows have a greater frame depth than the old ones being replaced.

The centre seal, just like the stop seal, is a type of rebate seal. These seals protect windows and doors against driving rain, noise and wind. The centre seal is fitted to the central rebate of the window profile. This seal is made of an elastic, durable rubber and reliably closes all the gaps between the window or door frame and the sash or leaf. The centre seal's special water-collection chamber ensures that any water that has penetrated or condensed is collected and channelled to the outside. The rebate seal plays an important role in optimising the window's U-value and improves noise protection. The centre seal of windows and doors, in particular, fulfils the function of keeping potential burglars out.

Modern windows and doors are constructed to keep the greater part of street noise outside. The sound insulation values (the sound reduction index) indicate how effectively the sound damping of a building component works.

The standard DIN 4109 "Sound insulation in building construction" sets out the requirements for sound insulation and defines the sound insulation classes. The sound insulation value is given in decibels (dB).

Example: the sound insulation value, or sound reduction index, of a window with insulating glazing is 32 dB Rw,P (or Rw p). The designation Rw denotes the reference value and P stands for the test value (tested in the laboratory). The sound reduction index of 32 dB Rw,P indicates that, in the laboratory measurements, the tested building component reduced the noise level by 32 dB.

In construction, the rule applies that the test value of a building component (Rw,P) should be 5 dB higher than the actual value (Rw). On average, a modern window with standard insulating glazing lets through only about 10% of the noise.

Sound insulation classes and the associated sound reduction index:

• Class 1 25-29 dB
• Class 2 30-34 dB
• Class 3 35-39 dB
• Class 4 40-44 dB
• Class 5 45-49 dB
• Class 6 over 50 dB

For living and working spaces, sound insulation class 1 is the standard; for schools, classes 2 and 3; for hospitals, 3 and 4.

Watertightness against driving rain describes the resistance of a building element (e.g. a window, glass wall or door) to driving rain, i.e. rainfall whose vertical direction of fall is altered by the wind. It is evident that the dynamic pressure exerted by driving rain is determined not only by the intensity of the rain itself, but also by the strength of the wind.

When windows and doors are tested for their watertightness against driving rain, the building elements are sprayed under a pressure of 0 to 600 Pa for between 15 and 55 minutes. The classification is made using numbers and letters, whereby the number from 1 to 9 corresponds to the test pressure and the letter (A or B) to the respective test method. A - unprotected windows and doors (e.g. not protected by a canopy), B - a partially protected type of construction.

Windows, glass walls and doors should also be able to withstand strong wind. The higher up in a building the corresponding component is installed, the greater the wind load. The wind load to which a building or its components are exposed also depends on the location of the building itself.

Wind load resistance is classified in accordance with the DIN EN 12210 standard: from Class 0 (not tested) to Class 5 (high resistance). In addition, three classes (A, B, C) are distinguished according to the relative frontal deflection under wind load. The final classification of components with regard to their wind load resistance is expressed using a letter-number combination, where the letter indicates the class of relative frontal deflection and the number indicates the wind load class.

Air permeability refers to the air exchange between the room and the outside. Windows and doors are never absolutely airtight. For optimal air quality in the rooms this is also not desirable. Excessive air exchange likewise reduces quality of life and, moreover, causes high heating costs.

Air permeability (formerly referred to as the a-value) is currently expressed by the Q-value and classified according to the standard DIN EN 12207. In the case of complete components (e.g. windows or doors), an area-related value can be given - the ratio of the air volume in m3 to the area per hour (m3/m2 times h). For the joints, a length-related value can be given - the ratio of the air volume to the length of the joint per hour (m3/m times h). The reference point is an air pressure of 100 Pa.

Multi-sash windows divide a large window area into individual sashes and can be implemented with various types of hardware. The window area can be subdivided either by a fixed central post (also known as a fixed mullion or transom) or by a loose false mullion (Stulp).

The Stulp (also referred to as a loose post) is the vertical profile strip on the false-mullion sash and, as an integral part of the window sash, opens together with the window. This creates an undivided window opening when the sash is open. When the false-mullion sash (also called the secondary sash) is closed, the Stulp profile serves as a rebate for the other sash.

A false-mullion window consists of two sashes joined in the centre. The main sash has a standard window handle, while the other has a hand lever (a release located in the frame).

The advantage of this window is that it can be opened "completely" without a central post remaining in place. False-mullion windows also provide a slim window construction and are particularly suitable for narrow window openings.

One disadvantage is that only the main sash of the window can be tilted. To open the secondary sash, you first have to open the main sash.

For wide windows (from approx. 150 cm), it is advisable to fit a post (transom) to improve the stability for the sashes to close against.

There are six resistance classes with increasing protective effect from RC1N to RC6 in accordance with the European standard DIN EN 1627 (the old designation, used until September 2011, was WK 1 to WK 6). RC stands for Resistance Class.

RC1N - Attempted break-in using physical force such as kicking, body-slamming, jumping against, levering up and the like. Elements of this resistance class are therefore mainly suitable as protection against vandalism. Windows of resistance class 1 offer only very low burglary protection.

RC2 - The opportunist offender additionally attempts to force the window open using simple tools such as a screwdriver, pliers or wedges. Windows of resistance class 2 offer low to average burglary protection.

RC3 - The offender additionally attempts to gain entry using a second screwdriver and/or a crowbar. WK3 windows offer high burglary protection. In a private household, protection to resistance class RC 2 and RC 3 is usually sufficient.

RC4 - The experienced offender additionally uses sawing and striking tools, e.g. an axe, crowbar, hammer and chisel, as well as a cordless drill. Windows of resistance class 4 offer very high burglary protection.

RC5 - The very experienced offender additionally deploys power tools, e.g. a drill, jigsaw or reciprocating saw and an angle grinder with a maximum disc diameter of 125 mm. Windows of resistance class 5 offer very, very high burglary protection. The resistance time is at least 15 minutes. 

RC6 - The very experienced offender additionally uses power tools such as a drill, jigsaw or reciprocating saw and an angle grinder with a maximum disc diameter of 230 mm. Windows of resistance class 6 offer extremely high burglary protection, and the resistance time is at least 20 minutes.  

The SWISSPACER warm edge spacer has become a byword for the effective prevention of thermal bridges on windows. It provides a special insulating effect at the edge of the insulating glass unit and determines the spacing between the panes. This prevents heat from escaping to the outside via the edge of the glass in winter and reduces the formation of condensation.

Windows have to meet ever more requirements, and development has advanced considerably over recent decades, particularly in the field of thermal insulation. Multi-pane insulating glass is one example of this. However, it can only deliver its full efficiency when a warm edge is built into the window by means of spacers. Triple glazing in particular benefits from an effective edge seal, which reduces energy loss to a minimum and supports the window's thermal insulation.

The DIN standards are a voluntary standard developed by the German Institute for Standardisation and are not directly binding. These recommendations are based on findings from technology, science and practical experience, and help to remove technical barriers to trade. Owing to the Europe-wide harmonisation of standards, revisions or updates are made time and again, including in the window sector.

German standardisation policy has an international focus. Only through their work at DIN do German experts gain access to the decision-making processes within the supranational standardisation bodies.

Standards developed at international level include, for example, ISO standards or the European EN standards.

The following section lists only a few of the DIN standards that can be found in the product descriptions on the website of Fensternorm GmbH:

• DIN EN 673 - Glass in building – Determination of thermal transmittance (U-value) – Calculation method
• DIN EN 410 - Glass in building - Determination of luminous and solar characteristics of glazing
• DIN EN ISO 10077 - Thermal performance of windows, doors and shutters - Calculation of thermal transmittance
• DIN EN 1627- Doors, windows, curtain walling, grilles and shutters - Burglar resistance - Requirements and classification
• DIN EN 13501-1- Fire classification of construction products and building elements
• DIN EN 1627 - Doors, windows, curtain walling, grilles and shutters - Burglar resistance - Requirements and classification

Originally, only small panes of glass could be manufactured, and glazing bars served as dividers. Nowadays, glazing bars are used predominantly for aesthetic reasons.

A distinction is made between three different types:

Genuine or glass-dividing bars

This refers to a type of bar that divides the glass, so that each bar field is fitted with an individual pane. Windows made up of several small panes are rarely found these days. This method of construction closely resembles the original way of manufacturing and is therefore also very labour-intensive. In addition, heat loss is greatest with the glass-dividing bar variant. Every interruption of the pane is in direct contact with the outside of the window, and this creates a cold bridge.

Imitation or bonded-on bars

The so-called “Viennese bar” (Wiener Sprosse) is counted among the imitation glazing bars and represents the affordable alternative to genuine glazing-bar windows. This type of bar is visually indistinguishable from glass-dividing bars and is ideal for recreating slim bars. The surface-mounted bars are simply bonded onto the glass and merely sealed with silicone. For this reason, they have no effect on the insulating properties of the glazing.

This type of bar looks particularly attractive on country-style houses or restored buildings that are listed as protected monuments. They give the entire building an exceptional and historic appearance.

Internal bars

Internal bars are inserted as a bar grid inside the glazing. This eliminates the labour-intensive upkeep of the bar construction. A particular feature of bars located within the cavity between the panes is that, with triple glazing, the bar is placed in only one of the two cavities between the panes.

Today, glazing-bar windows are used predominantly in the renovation of listed buildings and, in new builds, serve more as a design feature. In doing so, they lend the window and the entire building a distinctive character.

At Fensternorm, you can choose as standard between internal bars in a width of 18 mm or 8 mm and surface-mounted bars in a width of 26 mm. Of course, we will endeavour to fulfil (almost) any other size requirement you may have.

The thermal transmittance coefficient, abbreviated U for unit of heat transfer (the unit of heat transfer), replaced the k-value in the course of European standardisation. Measuring the U-value determines the heat loss from the inside to the outside, specifically per square metre and per hour at a 1° temperature difference.

A window's thermal insulation depends on the frame material, the glazing and its airtightness. A window's insulating properties are determined by the U-value. The lower this figure, the better the thermal insulation.

For windows, three U-values are generally distinguished. The thermal transmittance coefficient Uw refers to the entire window. This value incorporates the U-values for the glazing Ug and the frame Uf.

• Uf = U frame – is the U-value of the window frame.
  The Uf-value is determined by measurement for the frame-and-sash combination. The visible width of the profile is taken as the area for calculating the Uw-value.

• Ug = U glazing – is the U-value of the glazing.
  The Ug-value depends on the type of gas filling in the cavity between the panes, the distance between the panes and the number of panes.

• Uw = U window – is the U-value of the entire window.

The thermal transmittance coefficient for windows is generally calculated for the standard window size of 1.23 m x 1.48 m. Please note that larger glass areas achieve better values, since glazing allows better U-values than the frame material.

Under OIB Guideline 6 of 2011, the Uw-value for standard glazing must not exceed 1.4 W/m2K. The German Energy Saving Ordinance (EnEV) of 2014 limits the thermal transmittance coefficient to 1.3 W/m2K for an existing building. Windows with a Uw-value no greater than 0.8 W/m2K are considered suitable for passive houses.